Refactor/curp client retry

[iGxnon]

Based on #590

Please briefly answer these questions:

• what problem are you trying to solve? (or if there's no problem, what's the motivation for this change?)

  Add retry layer for curp client

• what changes does this pull request make?

  As above

• are there any non-obvious implications of these changes? (does it break compatibility with previous versions, etc)

  No

[codecov]

Codecov Report

Attention: 714 lines in your changes are missing coverage. Please review.

Comparison is base (fa656ad) 64.40% compared to head (f62a6d3) 76.64%.
Report is 38 commits behind head on master.

Files	Patch %	Lines
curp/src/client_new/unary.rs	0.00%	398 Missing ⚠️
curp/src/client_new/retry.rs	0.00%	115 Missing ⚠️
curp/src/rpc/connect.rs	0.00%	76 Missing ⚠️
curp/src/client_new/mod.rs	0.00%	67 Missing ⚠️
xline/src/server/kv_server.rs	31.25%	22 Missing ⚠️
curp/src/rpc/mod.rs	84.48%	9 Missing ⚠️
xline-client/src/clients/kv.rs	78.57%	6 Missing ⚠️
xlineapi/src/command.rs	64.70%	6 Missing ⚠️
xline/src/server/command.rs	70.00%	3 Missing ⚠️
curp/src/client.rs	86.66%	2 Missing ⚠️
... and 8 more

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #584       +/-   ##
===========================================
+ Coverage   64.40%   76.64%   +12.24%     
===========================================
  Files         105      157       +52     
  Lines       18802    21927     +3125     
  Branches    18802        0    -18802     
===========================================
+ Hits        12109    16806     +4697     
+ Misses       5955     5121      -834     
+ Partials      738        0      -738     

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[Phoenix500526]

LGTM

[themanforfree]

LGTM